The invention relates generally to an arrangement for and a method of determining the amount of dampening agent on a printing-image carrier.
More particularly, the invention relates to an arrangement for and a method of determining the amount of dampening agent on a rotating printing-image carrier during a printing operation, especially a printing-image carrier of an offset printing press.
A known arrangement for determining the amount of dampening agent on a rotating printing-image carrier in an offset printing press has a radiation source which directs pulses of radiation at a measuring spot or region of the carrier. The radiation includes radiation of a wavelength which at least approximates the absorption wavelength of the dampening agent. Radiation returned from the carrier outside of the angle of reflection of the radiation directed at the carrier is guided through a photoelectric transducer. The latter quantitatively converts the returned radiation pulses into electrical signals. These signals are processed in a processing circuit to yield a value representative of the amount of dampening agent which is present on the carrier.
An arrangement of this type is disclosed, for example, in the German Pat. No. 1 303 819. The arrangement of the German Patent employs radiation pulses in which the wavelengths lie within an absorption band of the dampening agent as well as additional radiation pulses in which the wavelengths are located in a range which is presumably only minimally affected by the dampening agent. The amplitudes of the pulses of different wavelength are differently affected by the dampening agent and quotients are formed from the amplitudes of the pulses of different wavelength. The quotients are represented by electrical signals which are processed in an attempt to draw conclusions regarding the amount of dampening agent present.
At the time the preceding arrangement was designed, it was recognized that the dampening agent affects not only the measuring pulses having wavelengths in the region of the absorption wavelength but also the comparative pulses in which the wavelengths are located outside of the absorption band. Accordingly, a measuring pulse as well as a comparative pulse must be registered during each revolution of the plate cylinder which carries the printing-image carrier so that the pulse amplitudes for each quotient are derived from the same layer of dampening agent. This can be accomplished in two ways. On the one hand, it is possible to have a time lag between generation of the measuring pulse and generation of the comparative pulse. However, when the measuring spot is small and the rotational speed of the plate cylinder is high, the required pulse frequency is hardly attainable with the current state of the art. On the other hand, it is possible to employ separate photoelectric transducers for the measuring pulse and the comparative pulse and to arrange appropriate different filters upstream of the respective transducers. In this case, there is the danger that errors in measurement which are not eliminated by the quotient procedure will arise due to the fact that the transducers have different characteristics, drifts and the like.
It has been further found that, while the effect of the dampening agent on a comparative pulse is small, it is not proportional to the amount of the dampening agent. Unexpected sharp variations occur at relatively large but nevertheless regularly encountered amounts of dampening agent. These variations make it virtually impossible to determine the amount of dampening agent using the conventional quotient procedure.